git-gui: Don't require a .pvcsrc to create Tools/Migrate menu hack

git-gui: Don't require a .pvcsrc to create Tools/Migrate menu hack

The Tools/Migrate menu option is a hack just for me.  Yes, that's
right, git-gui has a hidden feature that really only works for me,
and the users that I support within my day-job's great firewall.
The menu option is not supported outside of that environment.

In the past we only enabled Tools/Migrate if our special local
script 'gui-miga' existed in the proper location, and if there
was a special '.pvcsrc' in the top level of the working directory.
This latter test for the '.pvcsrc' file is now failing, as the file
was removed from all Git repositories due to changes made to other
tooling within the great firewall's realm.

I have changed the test to only work on Cygwin, and only if the
special 'gui-miga' is present.  This works around the configuration
changes made recently within the great firewall's realm, but really
this entire Tools/Migrate thing should be abstracted out into some
sort of plugin system so other users can extend git-gui as they need.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>